add note on beta-to-be

Author: quantizor

README.md

@@ -12,7 +12,7 @@ This plugin is a highly recommended supplement to the base styled-components lib
 Install the plugin first:
 
 ```
-npm install --save-dev babel-plugin-styled-components
+npm i -D babel-plugin-styled-components
 ```
 
 Then add it to your babel configuration:
@@ -23,6 +23,16 @@ Then add it to your babel configuration:
 }
 ```
 
+## styled-components v4 beta
+
+If you are using the styled-components v4 beta, you'll also need to use the beta version of this plugin:
+
+```
+npm i -D babel-plugin-styled-components@beta
+```
+
+Older versions should technically work, but some of the bundle size reduction benefits are only realized with the newer code!
+
 ## Documentation
 
 **The documentation for this plugin lives on [the styled-components website](https://www.styled-components.com/docs/tooling#babel-plugin)!**